The present invention relates to the production of filter means, and relates more particularly to tobacco smoke filter elements. More specifically, the instant inventive concepts are primarily concerned with producing filter means for cigarettes, although the products of this invention are generally useful as filters, particularly for tobacco smoking means, whether they be cigarettes, cigars, pipes or the like. Since filters for cigarettes are particularly commercially important, the basic embodiments of the instant invention will be discussed as they relate to the production of filtered cigarettes.
In making filters for use in connection with cigarettes and the like, a number of different properties of the resultant filter must be taken into consideration. While filtration efficiency, i.e., the ability of the filter to remove undesirable constituents from tobacco smoke, is perhaps the most important property of cigarette filters, filtration efficiency must frequently be compromised in order for the filter to possess a commercially acceptable combination of other properties, including pressure drop, taste, hardness, appearance and cost. For example, the most commonly utilized cellulose acetate filter has a relatively low filtration efficiency since increased efficiency can only be obtained either by increasing the density of the filter material or the length of the filter element, both of which produce a pressure drop across the filter which is excessive and unacceptable from a commercial standpoint.
In recent years, air dilution has become a popular technique for compensating for the relatively low filtration efficiency of cigarette filters having a sufficiently low pressure drop for commercial acceptance. The air dilution technique employs ventilating air to dilute the smoke stream from the cigarette and thereby reduce the quantity of tar and other undesirable tobacco smoke constituents drawn into the smoker's mouth for each puff or draw. The ventilating air is generally provided through a plurality of perforations in the tipping paper employed for joining the filter to the tobacco column of the cigarette, and if the filter is overwrapped with plug-wrap paper, an air pervious plugwrap paper is employed.
The air dilution technique has several advantages in that it is the most economical method of reducing tar, it enables achievement of the exact amount of tar delivery desired, and it also contributes to the removal of undesirable gas phase constituents, such as CO and NO. Disadvantages of the air dilution technique include lack of taste and uneven staining of the filter. Also, even though air dilution is an excellent means of providing for vapor phase removal, there is a need for selectively removing CO in relation to the tar content.
Previous methods for removal of the vapor phase have included the method described in British Patent Application Ser. No. 2,012,554 published Aug. 1, 1979, wherein the filter surface area is reduced by placing a thick plastic coating over a small acetate rod.
By the present invention there is provided an improved cigarette filter or the like which reduces the carbon monoxide (CO) content in relation to tar below any of the currently employed dilution techniques, and also leaves a clean appearance at the end of the filter after the cigarette has been smoked, while at the same time enhancing the taste properties of the filter. The improved filter of the present invention is obtained by sealing off the center portion of the filter and directing all of the smoke to the periphery of the filter. The sealing of the center portion of the filter may be accomplsihed by extruding a thin wall tube of a material which is impermeable to smoke and crimping this tube so as to seal one portion for every filter tip. The crimped tube is then overwrapped with a smoke permeable material such as cellulose acetate tow. Air is then blended with the smoke rather than the conventional method wherein the air travels down the outside of the filter leaving the smoke to come in through the center. By directing all of the gas phase to the periphery of the filter, the gas phase is exposed to the air dilution holes, causing the loss of CO and other undesirable gases.
In the use of a crimped tube, the beneficial results obtained by directing the smoke to the periphery of the filter are obtained primarily in the uncrimped portion where the smoke is directed to the outer periphery of the filter, rather than in the crimped area where the outer smoke permeable member will have a relatively large cross-sectional area. Also the air dilution holes are located outwardly of the uncrimped portion of the smoke impermeable inner member, as shown in the examples described hereinafter.
In addition to directing the smoke to the periphery of the filter, the amount of surface area through which the smoke travels is reduced, thus reducing the filtration efficiency of the tar removal. It has been found that low filtration filters, in conjunction with the air dilution feature, deliver more taste at the same tar levels, provided the draw resistance can be maintained. The theory on which this is based is that unfiltered smoke offers more taste than filtered smoke. By reducing the filtration and blending the air with all of the smoke, the same weight of tar is achieved but with improved taste. In addition, this method of completely blending the smoke can be designed so that it will provide the filter with a clean appearance after smoking and so that, in any event, the staining will be uniform.
A most important feature of the present invention is that a tar/CO ratio of approximately 2 is achieved, compared to a ratio of 1 for conventional air diluted cigarettes which deliver from 5 to 10 mgs. of tar. An additional important feature of the present invention is that the present filter can be produced at a lower cost than conventional acetate filters.
Other advantages of the tobacco smoke filter of the present invention include the fact that different shaped tubes may be extruded in order to provide various shapes for the filter. By modifying the crimp, many unique end appearances for the filter can be achieved. Also, by modifying the process and using white tubing, for example, a conventional end appearance of the filter can be maintained. In addition, a thick sheet of filter material can be employed rather than the acetate tow, by use of the wrapped filter-making technique so that the crimped filter tube would be wrapped with a filter material.
An additional feature of the present invention is the use of a plug of acetate tow material to prevent the thin wall crimped tube from being shown at the end of the filter.
Another feature of the present invention is a progressive ventilation feature, wherein the thin walled tube is provided with a series of perforations around its periphery in order to obtain a more uniform amount of tar passing through the filter.
Yet another feature of the present invention is the use of a filter comprising a reconstituted tobacco member, wherein the tobacco member is hidden from being shown at the end of the filter by the use of a plug of acetate tow.